A fuel cell is a kind of power generator capable of producing electric energy by electrochemically oxidizing fuels such as hydrogen and methanol, and a fuel cell has recently been noted as a clean energy supply source.
A fuel cell is classified into a phosphoric acid type, a molten carbonate type, a solid oxide type and a solid polymer electrolyte type according to the kind of electrolyte used. Among these fuel cells, a solid polymer electrolyte fuel cell is expected to be widely employed as a power supply for electric cars because of its low operation temperature, of 100° C. or lower, and its high energy density.
The solid polymer electrolyte fuel cell is basically composed of a solid polymer electrolyte membrane and a pair of gas diffusion electrodes connected to both surfaces of the solid polymer electrolyte membrane, and generates electricity when hydrogen is supplied to anodic electrode, oxygen is supplied to cathodic electrode and an external load circuit is connected between the electrodes. More specifically, protons and electrons are produced at the anodic electrode and protons transfer in the solid polymer electrolyte membrane to reach the cathodic side electrode, where the protons react with oxygen to produce water. On the other hand, electrons flowed out from the anodic side electrode through a conductor reach the cathodic side electrode through the conductor after losing electric energy in the external load circuit, and electrons contribute to the progression of the water production reaction.
As characteristics required to the solid polymer electrolyte membrane, high ionic conductivity is exemplified, first. It is considered that protons are stabilized by hydration of water molecules when protons transfer through the solid polymer electrolyte membrane. Therefore, strong hygroscopic properties, ionic conductivity and water dispersibility are important characteristics required of the solid polymer electrolyte membrane. As the solid polymer electrolyte membrane functions as a barrier which prevents the direct reaction of hydrogen with oxygen, low gas permeability is also required.
Examples of other required characteristics include the chemical stability required to endure a strong oxidative atmosphere during the operation of the fuel cell, and the mechanical strength required to endure a further reduction in thickness.
As the material of the solid polymer electrolyte membrane used in the solid polymer electrolyte fuel cell, a fluorinated ion-exchange resin has widely been employed because high chemical stability is required. Among these resins, for example, there has widely been employed “Nafion®” manufactured by DuPont, which has a main chain made of a perfluorocarbon and also has a sulfonic acid group at the end of a side chain. The fluorinated ion-exchange resin has generally well-balanced characteristics for the solid polymer electrolyte membrane. As the cell is put into practical use, higher durability has been required.
Methods of adding a polymer comprising a nitrogen-containing heterocyclic compound as one of polyazole-based compounds to a perfluorocarbon-based ion-exchange resin have been studied (see, for example, Korean Unexamined Patent Publication No. 2003-32321, International Publication WO 99/544707, and International Publication WO 98/07164). Furthermore, in Japanese Unexamined Patent Publication No. 2001-514431, there is a description which suggests that a structure interpenetrating polymer network is formed by dissolving a perfluorocarbon-based resin and a polymer serving as a porous base material in a common solvent and casting the resulting solution to form a membrane. As examples of the porous base material, polybenzoxazole or polybenzimidazole, as kinds of polyazoles are known.
These nitrogen-containing heterocyclic compounds can improve the mechanical and the thermal stability and it is expected that the durability during the operation of the cell will be improved. However, according to these studies, an aprotic solvent is used as a solvent to dissolve a perfluorocarbon-based ion-exchange resin or a nitrogen-containing heterocyclic compound. International Publication WO 98/07164 describes an aprotic solvent and water as an example of the solvent which is optionally used. However, there is not described that the solvent is used together with an alkali metal hydroxide. An electrolyte membrane for solid polymer electrolyte fuel cell made by using these aprotic solvents is generally inferior in dispersibility of a nitrogen-containing heterocyclic compound and the amount of the nitrogen-containing heterocyclic compound which effectively functions decreases, and thus high durability cannot be attained. When an acidic polymer solution is mixed with a basic polymer solution, a slightly soluble large-sized precipitate is quickly formed, thus making it difficult to obtain a uniform and clear solution. Therefore, a uniform membrane cannot be obtained.
Although the nitrogen-containing heterocyclic compound is dissolved in a bipolar aprotic solvent such as dimethylacetamide, high temperature is required so as to dissolve the compound, thus requiring a dissolution operation in a pressure-resistant vessel.
When a solid polymer electrolyte membrane is made by using the above aprotic solvent, the aprotic solvent is decomposed in case of preparing a polymer electrolyte-containing solution for membrane formation or forming a membrane from the polymer electrolyte-containing solution, and the resulting decomposition product is bonded with an ion-exchange group and, thus, the generating capacity of a fuel cell is lowered. Also the decomposition product remains in an exchange membrane and a fuel cell comprising a solid polymer electrolyte membrane containing the residual decomposition product assembled therein requires a long time until electric power is stably produced after starting. During the starting, the decomposition product is bonded with a catalyst in the electrode and gives a poisoning effect on a catalyst thereby to cause a problem such as low generating capacity of the fuel cell, and thus high durability cannot be obtained.
Dimethyl-sulfoxide (DMSO), dimethylacetamide (DMAC) and dimethyl-formamide (DMF), which have conventionally used as the aprotic solvent, per se, are harmful to the human body and is not considered to be friendly to the global environment. Furthermore, since these aprotic solvents have high boiling points, it takes a long time for purification and a high temperature is required for removal of the solvent, thus causing a problem such as poor productivity of the solid polymer electrolyte membrane. Also, these aprotic solvents are expensive.